Cardiac pacemakers and defibrillators to be implanted inside the human body require associated power supplies which must be provided with a high capacitance in order to be able to deliver intense bursts of current for very short time intervals on demand. That electrolytic capacitors are well suited for performing this function in biomedical electronic devices such as pacemakers and defibrillators is well known. Given the environment within which such a device is used, however, it is essential that the volume of the device be kept to an absolute minimum. Thus, since the capacitor in such a device ordinarily occupies as much as about 30% of the total volume of the device, which is a very high proportion relative to the other electronic components in the device, considerable effort has been expended on the problem of reducing the size of the capacitor as the best way for achieving a reduction in the size of the device as a whole. Nevertheless, attempts to minimize the volume of electrolytic capacitors have met with only limited degrees of success, for a number of reasons.
Conventionally, a capacitor of this type includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed Kraft paper or fabric gauze spacer impregnated with a solvent-based liquid electrolyte. The entire laminate is rolled up into the form of a substantially cylindrical body and encased, with the aid of suitable insulation, in an aluminum tube which is enclosed with the other electronics in a hermetically sealed case of a suitable metal (such as titanium, for example) inert to body fluids. However, Kraft paper or gauze fabric are inherently relatively thick. Thus, the presence of those materials will control the ultimate thickness of the rolled up laminate constituted by the anode, the cathode and the paper or gauze spacer, i.e., it will limit the extent to which the size of the capacitor can be reduced.
In any such electrolytic capacitor, of course, there exists the risk that the liquid electrolyte will leak out. Accordingly, the capacitor must be hermetically sealed to prevent any leakage of the liquid electrolyte therefrom, since if the liquid were to come into contact with the other electronic components encapsulated in the device, it could damage them sufficiently to cause the device to fail to operate properly. In extreme cases, the patient's life could then be in jeopardy. Hermetically sealing the liquid electrolyte into the capacitor thus has become standard practice, but this also inherently seals in any gases that may become liberated during the use of the capacitor. To accommodate such gases and prevent a potentially harmful buildup thereof, it has become necessary to provide the capacitor with an expansion or compliance chamber into which the gases can be permitted to escape and accumulate so as to avoid their having any adverse effect on the device. That, however, has entailed an increase, rather than a reduction, in the volume of the capacitor and is clearly an unacceptable expedient for use in a device for which minimization of volume is a critical consideration.
The presence of the liquid electrolyte in such a capacitor entails yet a further disadvantage. As is well known, the face of the aluminum anode is coated with a thin layer of aluminum oxide, which constitutes the dielectric for the capacitor and is formed through an electrochemical action resulting from the application of a positive voltage to the anode. The continued contact of the oxide layer with the solvent-based liquid or gel electrolyte over a period of time, however, especially while the capacitor is not in use, tends to cause the oxide layer to become degraded or "deformed" by being dissolved in the electrolyte, as a consequence of which the shelf life of the capacitor is relatively limited. Ordinarily, of course, the application of a voltage across the capacitor during use would tend to cause the oxide layer to be re-formed, however, the presence of the liquid electrolyte reduces the lifetime of the formed oxide layer. Thus, such a capacitor, in addition to a decreased shelf life, tends to have a shortened useful service life as well.
Among the attempts to achieve a reduction of the volume of such electrolytic capacitors is one represented by U.S. Pat. No. 3,555,369, which suggests the replacement of the conventional Kraft paper spacer or insulator of the capacitor with a thin semipermeable membrane of a polymeric material. Such a membrane would be thin, i.e., less than 40 .mu. thick, and preferably its thickness would be between about 1 .mu. and 2 .mu. or even less. Viewed in the abstract, this proposal might well have enabled a substantial reduction in the volume of the capacitor to be achieved because, given the normal thickness, on the order of about 100 .mu. or so, of the aluminum foil components of the capacitor, the size of the rolled up laminate would in essence be determined by the thickness of the foils, with the contribution of the semipermeable membrane layer to the overall thickness being, for all practical purposes, negligible. However, a capacitor according to this proposal requires that the semipermeable membrane must be impregnated with a solvent-based liquid electrolyte. Thus, the electrolytic capacitor of this patent must still be sealed hermetically in order to prevent any leakage of the electrolyte from the capacitor, and that in turn necessitates the provision of an expansion or compliance chamber to accommodate any liberated gases. The provision of such a chamber, of course, negates the volume reduction achieved by the use of the thin spacer constituted by the semipermeable membrane. Further, the presence of the liquid electrolyte in the electrolytic capacitor according to this patent will subject the capacitor to the previously described deformation of the oxide dielectric layer on the anode, and at the same time the presence of the liquid electrolyte will tend to adversely affect the lifetime of the formed oxide layer of the capacitor.
Starting from another vantage point, it has been proposed in U.S. Pat. No. 3,883,784 to produce capacitors in which the spacer or insulator between the anode and the cathode does not include a liquid or gel electrolyte but rather is at least in part a solid "polymeric association product" which, as disclosed in the patent, is a class of polymeric materials characterized by a multiplicity of ionic acceptors and a multiplicity of ionic donors (or interstitial impurities which act as ionic donors). The polymeric material is preferably an association product of polyethylene oxide (providing proton acceptor hydrogen bonding sites) and a polymeric resin such as a phenolic compound (providing proton donor hydrogen bonding sites), and it is suggested in the patent that this material may behave, in many aspects, like a solid electrolyte.
U.S. Pat. No. 3,883,784 discloses that the polymeric association product either may be impregnated into a conventional Kraft paper spacer before the latter is assembled with the metallic anode and cathode, or may be formed as a layer or film interposed (without any associated layer of paper) between the anode and cathode. However, apart from the case of a capacitor with a Kraft paper spacer (which is inherently subject to the limitation on capacitor volume reduction previously referred to herein), the patent further discloses that a film or layer of the polymeric association product when used as the spacer in a capacitor is on the order of about 0.0045 inch to about 0.0085 inch thick (approximately 114 .mu. to 216 .mu.). Thus, the polymeric association product spacers which are described in this patent are far thicker than conventional Kraft paper spacers, and consequently will not only fail to achieve a volume reduction for the capacitor but actually will tend to make the same larger than one utilizing a Kraft paper spacer.
Moreover, notwithstanding the suggestion that some of the various types of capacitors described in U.S. Pat. No. 3,883,784 may act like electrolytic capacitors in certain cases, they are clearly not electrolytic capacitors as that term is understood in the art and do not have the properties of those types of electrolytic capacitors which are suited for use in biomedical electronic devices such as pacemakers and defibrillators. This conclusion is implicit in the fact that the capacitors described in the patent and utilizing a spacer film made of the stated polymeric association product material may be bidirectional rather than polar devices. Thus, such a spacer film will then not be capable of supporting normal electrolytic action at any overvoltage, and placing a high negative voltage on the anodized aluminum electrode will reduce the oxide layer, producing aluminum and, in the presence of the hydrogen ions, hydroxyl ions, all without the capacitor having any substantial oxide layer reforming capability. Also, the capacitance values characterizing the capacitors described in the patent are much smaller than those of normal electrolytic capacitors of comparable size. Finally, the DC conductivity of the polymeric association product material used in forming the spacer films of those capacitors is extremely low for any material ostensibly functioning as an electrolyte.